1. Field of the Invention
This invention relates to laminated printing blankets and a method of manufacturing such blankets. The type of blanket referred to herein is compressible and is used primarily in offset lithographic printing, but may also find utility in other fields of printing.
2. Prior Art Statement
The use of blankets in offset lithography is well known and has a primary function of transferring ink from a printing plate to paper. Printing blankets are very carefully designed so that the surface of the blanket is not damaged either by the mechanical contact of the blanket with the parts of the press or by chemical reaction with the ink ingredients. Repeated mechanical contacts cause a certain amount of compression of the blanket which must be within acceptable limits so that the image is properly reproduced. It is also important that the blanket have resiliency, that is, be capable of returning to its orginal thickness, and that it provide constant image transfer regardless of the amount of use to which it is put.
Printing blankets are normally composed of a substrate base material which will give the blanket integrity. Woven fabrics are preferred for this base. The base may consist of one, two, three, or more layers of fabric. The working surface, by which is meant the surface that actually contacts the ink, is usually an elastomeric layer which may be made of natural or synthetic rubber which is applied over the base layer. This is usually done by calendering or spreading rubber in layers until a desired thickness of rubber has been deposited, after which the assembly is cured or vulcanized to provide the finished blanket. Such a blanket is acceptable for many applications, but often lacks the necessary compressibility and resiliency needed for other applications. It is desirable, therefore, to produce more highly compressible blankets with improved resiliency.
It is difficult to obtain an improved compressibility factor by the standard construction described above because the rubber material, while it is highly elastomeric, is not volume compressible, and cannot be compressed in a direction at right angles to its surface without causing a distortion or stretch of the blanket in areas adjacent to the point of compression. Therefore, if irregularities exist in the printing plate, the presses, or the paper, the compression to which the blanket is exposed will vary during the printinhg operation, and the irregularities in the plates, presses and paper will be magnified by the lack of compression in the printing blanket.
It has been found that by including at least one layer of material comprising a compressible layer of resilient polymer in a printing blanket, that printing problems such as those described above, as well as "blurring" (lack of definition), caused by a small standing wave in the blanket printing surface adjacent to the nip of the printing press, can be avoided. Also, a compressible layer can serve to absorb a "smash", that is a substantial deformation thereof caused by a temporary increase in the thickness in the material to be printed, for example, by the accidental introduction of more than one sheet of paper, or the like, during the printing operation. By incorporating a compressible layer in the blanket, a "smash" can be absorbed without permanent damage to the blanket or impairment of the printing quality of the blanket. In addition, a resilient compressible layer helps to maintain the evenness of the printing surface and the thickness of the blanket after compression at the nip of the press.
Many different means of producing a compressible layer within a printing blanket are known in the art. For example, compressible layers have been formed by mixing granular salt particles with a polymer used to produce the layer, and thereafter leaching the salt from the polymer to create voids. Voids in the compressible layer make possible positive displacement of the surface layer without distortion of the surface layer since volume compression occurs and displacement of the surface layer takes place in the direction of the voids, substantially perpendicular to the impact of the press. Other methods of creating a compressible layer, such as using blowing agents, and compressible fiber structures have been used in the prior art.
For example, it is known in the art to produce a compressible printing blanket by adding and mixing particles of hydrated magnesium sulphate in the elastomeric matrix of the layer, creating a blowing effect, and leaching the particles from the matrix, producing a compressible layer which has cavities and which are interconnected by passages, as is disclosed by Haren et al, in U.S. Pat. No. 4,025,685.
In the prior art methods, the forming of the voids using blowing agents, for example, has the disadvantage that the size of the voids formed and the interconnecting of the voids is not easily controlled. Oversized voids and interconnected voids cause some areas of the printing blanket to be more compressible and less resilient than adjacent areas of the printing blanket, which results in deformations during printing. Also, the gases produced by the blowing agents sometimes wick or leak into the surface layer or the base layer of the blanket, thereby weakening or affecting the quality of the printing blanket.
One method that has been proposed to alleviate this problem is to apply pressure to the compressible layer as it is being formed as is disclosed by Rodriquez, in U.S. Pat. No. 4,303,721.
The method and construction described by Rodriguez, however, is relatively expensive to use. Also, the need for a grinding step after the foaming of the nitrile rubber makes the method described by Rodriguez labor intensive.